The present invention relates to a continuous process for the preparation of unsaturated acetals by reacting olefinically unsaturated aliphatic compounds with allyl alcohols in a reaction column, where the reactants are only partially reacted in the reaction column, the resulting acetal is concentrated in at least two successive evaporation stages, and the recovered reactants are returned to the reaction column.
The preparation of unsaturated acetals by reacting only olefinically unsaturated aliphatic compounds with allyl alcohols in a reaction column in the presence of a distillable acid is known per se from DE 26 25 074. There it is described that a mixture of at least 2 mol of the alcohol and one mole of the aldehyde are to be introduced into the reaction column, and the water formed during the reaction is to be distilled off overhead and stripped off using a phase separator. The acetal is then to be removed as a crude product from the evaporator of the column. The reaction column should be operated such that aldehyde is no longer present in the still discharge from the column, i.e. the aldehyde is reacted completely in the reaction column. In the examples, conversions for the aldehyde of more than 94.5% are mentioned.
The process described represents a significant advance for the preparation of acetals, but unfortunately has a number of disadvantages. Since, according to the known prior art, high conversions of more than 90% are strived for, the production plant can only be regulated with difficulty. Slight variations in the amount of feed streams or in the purities of the feed substances mean that the desired conversion in the reaction column cannot be maintained. This circumstance is particularly notable when contaminated feed substances are used. Such impurities are e.g. secondary products of the aldehyde and of the alcohol, such as, for example, formates of the alcohol, ethers formed from the alcohol, or condensation products from alcohol and aldehyde linked via Cxe2x80x94C bonds. As is known, such by-products form in a relatively large amount particularly when the acetal is subjected to a cleavage and rearrangement reaction in accordance with Claisen and Cope, as is the case, for example, during the preparation of citral.
The addition of the correct amount of acid has proven particularly difficult. Even a slight deficit of the acid can lead to breakdown of the conversion in the reaction column. However, the addition of more than the correct amount of acid leads to a considerable increase in the amount of high-boiling secondary components and ethers. Regulation of the amount of acid is made more difficult by the fact that the acid accumulates in the reaction column and, as a result, the addition of too much or too little is only noticed after a considerable delay in time.
The abovementioned disadvantages have hitherto prevented the economic preparation of the acetals in the reaction column on an industrial scale.
It is an object of the invention to overcome said disadvantages and to provide a process for the preparation of unsaturated acetals which is easy to regulate and can be operated in a stable manner.
We have found that this object is achieved according to the invention by a process for the preparation of unsaturated acetals of the formula I 
in which
R1 to R7 independently of one another are hydrogen, a straight-chain or branched, optionally substituted C1-C6-alkyl radical and
R8 is hydrogen, a saturated or a mono- or polyunsaturated, straight-chain or branched, optionally substituted C1-C12-alkyl radical or an optionally substituted 3- to 12-membered saturated or a mono- or polyunsaturated carbocycle,
by reacting one mole of an aldehyde of the formula II 
in which R1 to R3 are as defined above, with at least 1 mol of an alcohol of the formula III 
in which R5 to R8 are as defined above, in the presence of catalytic amounts of acid and with removal of the water formed during the reaction, which comprises only partially reacting the reactants in a reaction column, concentrating the resulting acetal in at least two successive evaporation stages, and returning the recovered reactants to the reaction column.
A straight-chain or branched C1-C6-alkyl radical is understood as meaning, for example, a methyl, ethyl, propyl, isopropyl, butyl, tert-butyl, pentyl or hexyl radical, preferably a methyl or isopropyl radical.
A saturated or mono- or polyunsaturated straight-chain or branched C1-C12-alkyl radical is understood as meaning, for example, a methyl, ethyl, propyl, isopropyl, butyl, tert-butyl, pentyl, hexyl, allyl, ethylene, propylene, butylene, isobutylene, pentenyl, hexenyl, heptenyl, octenyl, decenyl, acetylene, propinyl, butinyl, butadienyl, isoprenyl or the hexadienyl radical, preferably the methyl or pentenyl radical.
A 3- to 12-membered saturated or mono- or polyunsaturated carbocycle is understood as meaning, for example, a cyclopropyl, cyclobutanyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctene, cyclohexene, cyclopentene, cyclooctadiene, cyclooctatetraene or a cyclododecatriene radical.
The substituents of a carbocyclic ring system or of an alkyl radical are understood as meaning, for example, halogen, nitro, cyano, hydroxyl, C1-C6-alkyl, C1-C6-alkoxy, C1-C6-alkoxycarbonyl or amino.
Alkoxy groups are, in combination with an alkyl group according to the above definition, with an oxygen atom, e.g. methoxy, ethoxy, propoxy, isopropoxy, butoxy or pentoxy, preferably methoxy.
Aldehydes of the formula II which may be used are, for example, acrolein, 2-buten-1-al, 2-methyl-2-buten-1-al, 3-methyl-2-buten-1-al, 2-methyl-4-methoxy-2-buten-1-al, 2-methyl-4-methoxy-2-buten-1-al, 3-isopropyl-2-buten-1-al. Of particular importance for further syntheses are acetals derived from 3-methyl-2-buten-1-al.
Alcohols of the formula III which are suitable according to the invention are, for example, 2-propen-1-ol, 2-buten-1-ol, 2-methyl-3-buten-2-ol, 3-methyl-2-buten-1-ol, geraniol, 2-methyl-2-propen-1-ol, preferably 3-methyl-2-buten-1-ol (prenol).
The acetals produced by the process of the invention are valuable starting materials for plastics, active ingredients, fragrances and vitamins. For example, 1,1-bis(3-methyl-2-buten-1-yloxy)-3-methyl-2-butene (3-methyl-2-butenal diprenyl acetal) is an important starting compound for the fragrance citral.